Multiple subnets with classes - 'option routers' issue

[Frank Bulk]

Most CMTS' define multiple relay agents for each type.

Here's my config snippet:

 cable helper-address a.b.c.d cable-modem global
 cable helper-address a.b.c.d host global
 cable helper-address a.b.c.d mta global

The primary address on the CMTS interface are the CMs, and so the DHCP
server sees that network (gi-addr?) and the CMs all get IP addresses from
that range.  The CPEs are relayed differently, and get IP addresses from my
shared network that has many ranges.

It just works.

Frank

-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On Behalf
Of Gundares
Sent: Saturday, June 07, 2008 11:16 AM
To: dhcp-users
Subject: Multiple subnets with classes - 'option routers' issue

Dear dhcp-users,

first, the essence - the isc-dhcpd provides the default router
configured in the first subnet declaration for clients from other
subnets.

Here are the details.

I'm using isc-dhcpd 3.0.5 under FreeBSD-6.3.

There are two CMTSes at the customer site with three (total) cable
interfaces, two IP-subnets on each interface (one for modems, another
for PCs). So, CMTSes are acting as dhcp-relays (they are routers).

Cable modems are getting IP and its config-files depending on classes
Classes are declared like this:

subnet 172.16.0.0 netmask 255.255.248.0 {

.. <some 'global' options common for all types of cable modems>
.. <i.e. time server, time offset, log server, etc>
option routers 172.16.0.1;
..
..


class "SB5100" {
match if (MOTO.modnum = "SB5100"); # this is suboption 9 of option 43
filename "sb5100.bin"
}
class "EPX2203"
match if (MOTO.modnum = "EPX2203";
filename "epx2203.bin";
}
pool {
allow members of "SB5100";
allow members of "EPX2203";
deny  members of "SB5100-UPG";
range 172.16.0.50 172.16.7.254;
}
pool {
deny  members of "SB5100";
deny  members of "EPX2203";
allow members of "SB5100-UPG";
range 172.16.0.10 172.16.0.49;
}
}

This dhcpd-config was working fine for many years (for only one CMTS),
but two days ago client bought the second one. So, I've added new
subnets with the same class-definitions (now dhcpd logs error: 'too many
classes match MODEM_MAC_ADDR'). After that, the cable modem connected
with the second CMTS (subnet 172.17.0.0/18), gets all the options
properly, except the default router option - 172.16.0.1 (this is the
first CMTS' cable interface primary IP-address).

tcpdump`ed OFFER packet looks like this:

2008-06-07 18:23:09.001689
IP (tos 0x0, ttl  64, id 53242, offset 0, flags [none],
proto: UDP (17), length: 336, bad cksum 0 (->fb94)!)
<MY_SERVER'S_IP>.67 > 172.17.0.1.67:
[bad udp cksum 648!] BOOTP/DHCP, Reply,
length: 308, hops:1, xid:0x167cd60d, flags: [Broadcast] (0x8000)
          Your IP: 172.17.31.254
          Server IP: <MY_SERVER'S_IP>
>>>>>>    Gateway IP: 172.17.0.1
          Client Ethernet Address: 00:1a:ad:71:1a:b4
          file "sb5100.bin"
          Vendor-rfc1048:
            DHCP:OFFER
            SID:<MY_SERVER'S_IP>
            LT:86400
            SM:255.255.192.0
            TZ:14400

>>>>>>      DG:172.16.0.1

            TS:<MY_SERVER'S_IP>
            LOG:<MY_SERVER'S_IP>
            NS:<MY_SERVER'S_IP>
            ACKT:1.4.128.1.0.3.2.6.0.26.173.113.26.180

So, how can I set the _proper_ default gateway address for clients?

--
Regards,
Ivan V. Borodin <ivan at vimcom.ru>
Computer Systems Engineer
JSC "Vimcom Optic TS"
p. +7(495)-737-37-57
f. +7(495)-737-37-55